Synthesis of Polyurea Thermoplastics through a Nonisocyanate Route Using CO₂ and Aliphatic Diamines

The transformation of CO₂ can alleviate environmental problems and promote sustainable development. We prepared polyureas (PUas) from CO₂ and diamines via an isocyanate-free route. PUas synthesized by the copolymerization of CO₂ with 1,6-hexamethylene diamine (HMDA) and 1,12-diaminododecane (DAD) presented superior mechanical properties compared to the ones from CO₂ with either DAD or HMDA alone. The PUas contained DAD and HMDA segments in the main polymer chains, and hydrogen bonds were formed between DAD and HMDA segments, in which the hydrogen bonds were distributed at regular and irregular intervals depending on the relative amounts of the two segments. For PUas synthesized from either DAD (<i>x</i> = 0) or HMDA (<i>x</i> = 100), hydrogen bonds were mostly present at regular distances, forming a regular hydrogen bond network and contributing to high crystallinity. For PUas of DAD_{100–<i>x</i>}HMDA_<i>x</i> (37 ≤ <i>x</i> ≤ 63), in contrast, irregular hydrogen bonds coexist with regular ones, so the crystallinity was lower compared to DAD₁₀₀ and HMDA₁₀₀. It is to note that the PUas synthesized are advantageous for postprocessing as their initial decomposition temperatures (<i>T</i>_<i>d</i>,5% > 295 °C) are about 85 °C higher than the melting temperatures (<i>T</i>_<i>m</i>, 116–207 °C). Furthermore, mechanical properties were optimized by varying the chain length of aliphatic diamine used. The PUas (DAD₅₀HMDA₅₀ and DAD₃₇HMDA₆₃) obtained show mechanical properties superior to those of the previously reported polymeric materials, such as CO₂-based PUas, isocyanate-free polyhydroxyurethanes, CO₂-sourced polycarbonates, polyamide 6, and traditional PUas from diisocyanates. Hence, the present CO₂-sourced PUas will find industrial applications in different fields.